Oftentimes, the formation of capacitors as part of semiconductor circuitry involves (1) forming an oxide layer over a semiconductive substrate, such as a silicon substrate; (2) forming a plate over the oxide; and (3) doping the active area around the plate. The plate/oxide/substrate combination acts as a capacitor, which can serve many uses in semiconductor circuitry. For example, when placed in electrical communication with the connection pads of a die, such a capacitor can be used to filter voltage transients which may be generated by lead frame connection wires that also lead to the connection pads. In such cases, it is beneficial to form a depletion mode capacitor, wherein, between steps (1) and (2), a depletion well is implanted under the plate site with dopants of the same type as the dopants that are to be used in step (3) but at a lesser concentration. As a result, the depletion well is depleted of current carriers (electrons or holes).
As another example, capacitors can be used as part of a charge pump, which provides a current at a potential greater than the potential initially supplied to the circuit. Charge pumps often comprise an oscillator coupled to a capacitor. The capacitor, in turn, is coupled to an output node leading to other circuitry, such as a memory array. As the capacitor receives a signal from the oscillator that rises in voltage, the charge held by the capacitor is pumped to a level above VCC (commonly referred to as VCCP). This charge is subsequently discharged to the output node and used to drive circuitry external to the oscillator.
In a third example, capacitors can be used in delay circuitry, such as RC delay circuits. In such circuits, a resistive element is interposed between an input node and an output node, and one plate of the capacitor is coupled to the output node and to the resistive element, with the other plate coupled to a reference voltage node. The output node's ability to respond to a change in voltage at the input node is delayed by the charging or discharging of the capacitor during that change.
As yet another example, capacitors can be incorporated into various types of regulator circuits such as reference voltage regulators. Examples of such can be found in U.S. Pat. No. 5,513,089 (FIG. 6 and accompanying text) and U.S. Pat. No. 5,581,206.
Regardless of the particular use for a capacitor, there is a constant need in the art to increase capacitance without increasing the die area used by the capacitor. It would also be desirable to maintain or even increase capacitance as the size of capacitors decreases in order to help fit more die on one wafer.
It is noteworthy that, in determining the capacitance of a storage device, prior art often focuses on the capacitance generated between the parallel portions of opposing plates, otherwise known as parallel capacitance. Nevertheless, it is also known that fringe capacitance exists at the sides of the plates and results from the non-uniform electric field at those sides. Further, it is known that this fringe capacitance can play a factor in increasing the total capacitance of a storage device. For example, in U.S. Pat. No. 4,931,901, two plates located side-by-side are added to the existing plates and are used to “fine tune” the total capacitance by allowing relatively small increases in the total capacitance of the storage device. As another example, U.S. Pat. No. 5,583,359, by Ng et al., discloses a capacitor having interleaved plates—wherein the material of one plate extends into a gap within the other plate. The capacitance between the extended portion of the first plate and the laterally adjacent sides of the other plate serves to increase the total capacitance to a point beyond that demonstrated by prior art capacitors with non-interleaved plates.
Another method for increasing capacitance is to create microstructures on the surface area of one of the electrodes, such as by forming hemispherical silicon grain (HSG) thereon. U.S. Pat. No. 5,554,557, by Koh, describes such a method. The bottom electrode consists of a conductive layer with HSG deposited thereon. In the disclosed LPCVD process, grains having a diameter of about 800 angstroms are formed on the conductive layer. (Koh at column 6, lines 45-56.) A dielectric is then conformally layered onto the HSG at a thickness ranging from 40 to 60 angstroms. (Koh at column 6 line 63-column 7 line 7.) Finally, the top electrode is formed over the dielectric. (Koh at column 7, line 8.) However, given the general size of the grains, the conformal nature of the overlying materials, and the thinness of the dielectric, the top electrode extends into the gap defined by the grains of the bottom electrode. As a result, capacitors using HSG are analogous to the interleaved plates discussed above.
In calculating the capacitance between non-interleaved plates, however, fringe capacitance is often disregarded in the prior art. See, for example, DAVID HALLIDAY & ROBERT RESNIK, FUNDAMENTALS OF PHYSICS 620 (1988 3d ed.).